The present invention relates to a method for determining a position of a mobile earth station for use in satellite communication systems such as fixed-satellite communication systems using a non-geostationary satellite, a mobile-satellite communication system and a personal-satellite communication system.
In recent years, there have been made many proposals and studies in the United States and European countries for commercial services using non-geostationary satellites intended primarily for mobile-satellite communication services.
In the United States, Motorola Inc. has proposed, with the aim of its commercial inauguration in 1998, an IRIDIUM project that implements a worldwide scale personal satellite communication system with a total of 66 LEO (Low Earth Orbiting) satellites flying at an altitude of about 780 km. Furthermore, there have been proposed in the United States a Global Star system by Qualcom Inc. that is a mobile-satellite communication system which continuously covers the whole world with a total of 48 LEO satellites flying at an altitude of 1,289 km and an Odyssey system by TRW Inc. which aims to set up a global network which covers nine main areas of the world, and moves toward realization of these systems are now growing more and more brisk.
On the other hand, European countries are also interested in these mobile satellite communication systems using non-geostationary satellites; especially, ESA (European Space Agency) has proposed and is now making study of an ARCHIMEDES project utilizing a highly elliptical orbit (HEO) that is suitable for areas in higher latitudes. Moreover, INMARSAT, an international mobile satellite communication organization, is proceeding briskly with research and development work on an INMARSAT-P system which is intended to provide, from 2000 up, a global-scale personal-satellite communication services via portable terminals through the use of 10 satellites placed in the medium earth orbit of an 10,355 km altitude. In the above-mentioned satellite communication systems using non-geostationary satellites, it is assumed that each portable terminal exists within the coverage of any spot beam of at least one non-geostationary satellite.
The mobile satellite communication system using non-geostationary satellites utilizes multi-spot beam satellites of the type that the beam coverage by one satellite is formed by a plurality of spot beams. In the multi-spot beam satellite, communication channels of different frequencies are usually allocated to adjacent spot beams so as to increase the efficiency of frequency by communication channels. Hence, in the mobile satellite communication system using non-geostationary satellites, the points listed below are important when getting position datas of users.
(1) To Minimize the Number of Spot Beams Which are Transmitted Over a Channel to Call a Mobile Earth Station.
That is to say, when the precise position of the mobile earth station to be called from a terrestrial network is unavailable, it is necessary that control signals for calling be transmitted over a wide beam areas. This impairs the utilization factor of frequency by the control channels, resulting in limitations being imposed on the maximum traffic volume of the control signals and transmitting power being wasted.
(2) To Utilize the Position Data on the Mobile Earth Station as Information Auxiliary to a Criterion for Starting a Spot Beam Handover and a Satellite Handover
That is to say, when a mobile earth station continues communication across a global beam coverage by two satellites or two different spot beam, it is necessary to effect a handover (switching of a satellite beam for communication) between spot beams or global beams; in this instance, if the precise position of the mobile earth station is available, it is possible to perform the handover while keeping the communication link in a high quality state. An error in the timing of starting the handover will result in a cutoff of the communication link or degradation of its quality.
(3) To Assign a Proper Land Earth Station at the Time of Connecting a Call Between a Mobile Earth Station and a Land Earth Station
That is to say, when a mobile earth station and a terrestrial network are connected via a land earth station, the service area coverage is allowed is limited to a certain range around the land earth station, but in the case of connecting a communication link between a mobile earth station and a land earth station, it is necessary to select a land earth station that maximizes the call duration. This requires precise position data of the earth station; if the precise position of the mobile earth station cannot be detected, then a wrong land earth station will be selected, incurring the possibility of the link being cutoff during communication.
(4) To Distinguish Borders.
That is to say, in a mobile-satellite communication system capable of providing global-scale personal communication services, it is possible to achieve communications from anywhere in the world, but in the case of establishing communications from an area near the border with a country where such global-scale personal communication services are prohibited, it is necessary to recognize the precise position of the mobile earth station and judge whether communications are possible or not.
(5) To Confirm the Position of a User in Emergency and Distress Call
That is to say, when a user is in a state of emergency such as an accident, if the precise position of the user can be detected on the part of a ground station, the user can be promptly rescued from danger.
(6) To Provide Additional Services (to Offer Position Data to Users)
If the precise position of a user, i.e. a mobile earth station can be detected, a position data service such as a navigation service can be offered to the user by presenting the information to the mobile earth station.
There have been proposed the following methods for determining a position of a mobile earth station.
(a) The position of the mobile earth station is estimated from area information of a mobile-location or existing-zone spot beam of the mobile earth station. In this instance, the position determination accuracy corresponds to the radius of the spot beam and the position of the mobile earth station can usually be estimated with an accuracy of hundreds to thousands of kilometers. PA1 (b) In a three-dimensional or geocentric coordinate system wherein the center of gravity of the earth is at the origin, the equatorial plane or zone is in the x-y plane, a longitude of zero degree is in the positive orientation of the x-axis and an east longitude of 90 degrees is in the positive orientation of the y-axis, the position of a mobile earth station can be detected by calculating points of intersection of circular surfaces about a satellite through the use of data on a distance between the satellite and the mobile earth station measured a plurality of times (twice or more), or calculating points of intersection of conical surfaces about the satellite through the use of data on the Doppler shift amount between the satellite and the earth measured a plurality of times (twice or more), or calculating points of intersection of spherical surfaces and conical surfaces about the satellite through the combined use of such data. In this instance, there are two points of intersection, and this phenomenon is called ambiguity of solution; usually, the true position of the mobile earth station can be uniquely determined from the area code of the spot beam corresponding to the mobile earth station. Furthermore, it is customary in the art to adopt a scheme that expresses the position of the mobile earth station as an unknown variable and calculates it by the least mean square technique, instead of directly seeking a solution to simultaneous equations. PA1 (c) The mobile earth station detects the position of its own through utilization of the existing satellite navigation or positioning system such as GPS (Global Positioning System or GLONASS (GLObal Navigation Satellite System), or such an existing radio navigation system as loran C, and sends the position data to the land earth station.
Incidentally, the distance between the satellite and the mobile earth station can be measured by providing both of a land earth station and the mobile earth station with timing-synchronized clocks, then measuring the propagation time of a signal sent from the land earth station or the mobile earth station in the latter or former, and multiplying the measured propagation time by the velocity of light. To measure the Doppler shift amount between the satellite and the mobile earth station, both of the land earth station and the mobile earth station are provided with frequency-synchronized frequency oscillators, then the amount of frequency offset of a signal sent from the land earth station or the mobile earth station is measured in the latter or former and the amount of frequency offset between the satellite and the mobile earth station is subtracted from the measured value, by which the amount of frequency offset or the Doppler shift amount between the satellite and the mobile earth station can be obtained.
Of the above-described mobile earth station position determination methods (a) to (c), it is the method (c) that is the most stable in positioning accuracy. With the method (b), the positioning accuracy is likely to vary materially from tens of meters to thousands of kilometers according to orbital parameters of the satellites used, the number of satellites that can be used for positioning, the distance to be measured, the accuracy of measuring the Doppler shift amount and the time intervals between individual position determinations. The method (a) can be applied only to rough positioning of the mobile earth station and, as a general rule, cannot be used for high accuracy positioning.
The above-mentioned mobile earth station position determining methods (a) to (c) for the satellite communication systems using non-geostationary satellites each have such problems as mentioned below. That is, in the method (a) of position determinating the mobile earth station on a spot-beam basis, the position determination accuracy scatters largely from hundreds to thousands of kilometers; hence, to ensure calling the mobile earth station from the terrestrial network side, it is necessary to do simultaneous transmission using all spot beams adjacent to the spot beam of the existing zone of the mobile earth station. This decreases the utilization factor of frequency by the signalling channel, presenting problems that the maximum traffic volume of the signalling traffic is limited and that the transmitted power is wasted. In a case where a mobile earth station needs to continue communication between spot beams or global beams, since the precise position of the mobile earth station is unavailable, it is necessary, for effecting the handover with the communication link in its high quality state, that the overlapping area of adjacent spot beams be extended to leave an extra link margin at the spot beam edge. As the result of this, spot beams cannot efficiently be located and expensive satellites with a high-output power transmitter are needed. Furthermore, an appropriate land earth station cannot be assigned at the time of call connection between a mobile earth station and a land earth station; hence, a wrong land earth station is selected and the link is likely to be disconnected during communication. Besides, this method can neither identify a border, nor confirm the position of a user in emergency and distress call, nor offer the navigation service.
Next, the method (b) has the following problems with the position determination procedure. That is, in the method of estimating the position of the mobile earth station in the geocentric coordinate system, appropriate values need to be set as initial values for the least mean square method so as to obtain the estimated position of the mobile earth station without divergence of the least mean square method. To meet with this requirement, however, it is necessary that all points on the earth surface expressed by the geocentric coordinate system be prepared as candidates for the initial values. If the initial values for the least mean square method are set wrong, the least mean square method converges to a different solution in which its error approaches zero, incurring the possibility of detecting an incorrect position of the mobile earth station. Moreover, when a plurality of solutions obtainable with the least mean square method are present in the same spot beam, it is impossible to apply the conventional method of identification according to the area code of each spot beam and the position of the mobile earth station cannot uniquely be determined. This phenomenon will hereinbelow be described with reference to FIG. 9. For the sake of brevity, however, the description will be given of a positioning scheme which measures twice the distance between a satellite and a mobile earth station through the use of one non-geostationary satellite.
FIG. 9 illustrates a scheme which performs position determination using the distances, r.sub.k (112) and r.sub.k+1 (113), between the satellite and the mobile earth station measured at times t.sub.k and t.sub.k+1. Intersections of circles, which are centered at intersections Q(t.sub.k (110) and Q(t.sub.k+1) (111) of a perpendicular dropped from the satellite to the earth surface and the latter at the times t.sub.k and t.sub.k+1 and have radii r.sub.k (112) and r.sub.k+1 (113), respectively, are obtained as two solutions A (116) and B (117) for the estimated position of the mobile earth station; in this instance, the two points A (116) and B (117) are symmetrically located with respect to a straight line n (115) passing through points Q (t.sub.k) (110) and Q (t.sub.k+1) (111). That is, one of the two points coincides with the true position of the mobile earth station, whereas the other is a falsely estimated position; in such a case, it is impossible to determine the truly estimated position from the results measured once. In consequence, as is the case with the scheme (a), the positioning accuracy of the mobile earth station scatters a largely as hundreds to thousands of kilometers. Hence, to ensure calling the mobile earth station from the terrestrial network side, it is necessary to perform simultaneous transmission for calling with all spot beams adjacent to the spot beam of the existing-zone of the mobile earth station, and accurate handover and the assignment of an appropriate land earth station cannot be done; furthermore, this scheme is incapable of identifying borders, confirming the position of a user in emergency and distress call and providing the navigation service.
In the case of the scheme (c), a receiver needs to be provided solely for utilizing the existing satellite positioning system or radio naviation system. This is likely to pose problems such an increase in the cost of mobile earth station terminal equipment, an increase in power consumption and limitations on the miniaturization of each terminal equipment. Besides, since the satellite positioning systems such as GPS and GLONASS are systems built primarily for military purposes, there is no absolute guarantee of receiving the positioning service at all times. Thus, it is considered to be risky that the position determining method of mobile earth stations depends on such conventional systems.